1. Field of the Invention
The present invention relates to magnetite particles. More particularly, the invention relates to magnetite particles which have a silicon component, an aluminum component, and a metal element other than iron, both at the center of and on the surface of the particles, and in which the amounts of the silicon component, the aluminum component and the metal element dispersed near the surface of the particle, and the amounts of the silicon component and the aluminum component exposed on the surface of the particle are controlled, whereby the properties of the magnetite particles, such as oil absorption, electrical resistance, magnetic characteristics, and environmental resistance, are improved in a well-balanced manner. Thus, the magnetite particles are mainly used as a starting powder for magnetic toners for electrostatic copying, and as black pigment powders for paints. The invention also concerns a process for producing such magnetite particles.
2. Description of the Related Art
Recently, magnetite particles produced by a reaction in an aqueous solution have been widely used as a material for a magnetic toner for electrophotographic copiers or printers. Various general development characteristics are required of the magnetic toner. With the progress of electrophotographic technologies in recent years, rapid advances have been made, particularly in copiers and printers employing digital techniques. Thus, more advanced properties have been required of them. In detail, outputs such as graphics or photographs have been required in addition to conventional character outputs. Among the printers, in particular, those capable of printing 1,200 dots or more per inch have appeared, making latent images on the photoconductor even more meticulous. Thus, there is an intense demand for high reproducibility of thin lines by development.
Japanese Unexamined Patent Publication No. 72801/93, for example, discloses a magnetic toner wherein a magnetic powder with high resistance and good flowability is desired.
In disclosures of magnetic toners made in Japanese Unexamined Patent Publication No. 100474/93, magnetic iron oxide is described as still requiring an improvement in environmental resistance. As disclosed there, a magnetic powder is desired which not only possesses the required properties for production of a toner, but is also excellent in environmental resistance (moisture resistance).
Likewise, Japanese Unexamined Patent Publication No. 239571/95 points out that the environmental resistance of a magnetic powder, especially under hot humid conditions, is problematical.
According to what is disclosed about a magnetic toner in Japanese Unexamined Patent Publication No. 1160/91, high resistance and low hygroscopicity are needed to show the required performance in diverse environments. To eliminate scatter over images, a magnetic toner should be low in coercive force and residual magnetization.
In addition, toner is classified into negatively chargeable toner and positively chargeable toner. Resin carriers comprising magnetite or the like also come in a negative charge type and a positive charge type.
In short, to fulfill the above-described requirements, it is necessary to provide a magnetic powder which has the usually required properties, which is well balanced in resistance, flowability, environmental resistance, and magnetic characteristics, and whose charge properties can be adjusted arbitrarily.
With this background, various improvements have been achieved in magnetite particles. Japanese Unexamined Patent Publication Nos. 155223/86, 278131/87, and 24412/87, for example, disclose magnetite particles containing a silicon component only inside the particle. These particles give an image quality with improved image density, but the image quality is still insufficient. Furthermore, the magnetite particles proposed by these publications are poor in flowability, and the packing density of the powder becomes too high because of vibrations during transportation, and so forth. This poses the problem of considerably lowering workability for production of a toner.
Japanese Unexamined Patent Publication No. 110598/95 discloses magnetite particles containing a silicon component inside the particle, and having the silicon component and an aluminum component precipitated and deposited on the surface of the particle as a co-precipitate. However, improvement achieved in the image density and the flowability are still insufficient. Besides, the surface of the particle is covered with the silicon and aluminum components. Thus, the environmental resistance due to moisture absorption is unsatisfactory, and many agglomerates appear.
Japanese Unexamined Patent Publication No. 170325/92 discloses a process for producing particles having high coercive force and high residual magnetization by coating the surface of particles, which contain or do not contain a silicon component and an aluminum component, with silicon or aluminum, followed by oxidation or reduction. However, this publication does not have as an object the provision of magnetite particles having low residual magnetization and low coercive force, having high electrical resistance, excellent in workability, flowability, dispersibility and environmental resistance, and arbitrarily adjustable in charging properties according to a purpose to be attained, and a process for producing such magnetite particles. The intermediate product before oxidation or reduction also has the same drawbacks.
Japanese Unexamined Patent Publication No. 213620/93 discloses magnetite particles containing a silicon component at the center and on the surface thereof, and having well balanced residual magnetization, satisfactory flowability, and high electrical resistance. These particles give an image quality with improved thin line reproducibility, but are problematical in environmental stability because of hygroscopicity.
Japanese Unexamined Patent Publication No. 267646/95 discloses the production of a magnetic powder excellent in resistance to moisture absorption and magnetic characteristics by the use of silicic acid anhydride. However, the silicon component used is silicic acid anhydride, which is added in the course of the reaction for forming particles. Thus, silicic acid anhydride is present as independent fine particles in the resulting magnetite particles. In addition, no silicon component is present at the center of the magnetite particle. Thus, residual magnetization is high, and flowability is still insufficient.
Japanese Unexamined Patent Publication No. 59025/97 discloses a magnetic powder having high flowability and blackness obtained by the use of silicon, etc. However, the coercive force is high compared with the particle size, so that the improvement in thin line reproducibility of images and the flowability are still far from satisfactory.